Packaging materials comprising an aluminum foil layer provided on a base material have hitherto been used as packaging materials having gas barrier properties. These packaging materials have stable gas barrier properties, but on the other hand, due to the provision of an aluminum foil layer as a barrier layer, the level of suitability for incineration is so low that the disposal after use is not easy. Further, since the packaging material is provided with the aluminum foil layer, disadvantageously, the packaging material is not transparent.
In order to solve such problems, a packaging material comprising a barrier layer formed of polyvinylidene chloride (PVDC) or an ethylene-vinyl alcohol copolymer (EVOH) has been developed.
Since, however, PVDC contains chlorine, incineration after use generates chlorine gas which is disadvantageous from the viewpoint of environmental hygiene. EVOH advantageously has a high level of oxygen gas barrier properties and has a low level of flavor compound adsorption, but on the other hand, the oxygen gas barrier properties are disadvantageously lowered under a high-humidity atmosphere. Further, EVOH involves an additional problem that it does not have water vapor barrier properties. To overcome these problems, a complicated laminated structure should be adopted in the packaging material for shielding the EVOH as the barrier layer from water vapor. This may disadvantageously lead to increased production cost.
In recent years, films comprising a barrier layer formed of a thin film of an inorganic oxide such as silicon oxide or aluminum oxide have been developed as packaging materials that stably exhibit a high level of gas barrier properties and a high level of aroma retaining properties and, at the same time, are transparent. This thin film of an inorganic oxide is formed by depositing the inorganic material onto a base material by vacuum vapor deposition. Accordingly, these films do not involve any environmental problem in the disposal and, further, there is no dependence of gas barrier properties upon humidity.
Since the barrier layer formed of a thin film of, for example, silicon oxide or aluminum oxide is formed by vapor depositing inorganic oxide particles on a base material, gaps called crystal grain boundaries exist among the inorganic oxide particles. Accordingly, the gas barrier properties of the thin film are unsatisfactory. In order to overcome this drawback, the film thickness should be increased (500 to 1000 angstroms). Increasing the film thickness, however, poses problems of poor malleability and high susceptibility to cracking.
The lower the proportion of oxygen atom in the inorganic oxide, the better the level of gas barrier properties. However, various problems occur such as a deterioration in transparency and poor adhesion between the substrate and the inorganic oxide particles.
To overcome the problems, a proposal has been made in which a coating film having gas barrier properties is provided on the vapor deposited film surface to improve gas barrier properties (Japanese Patent Laid-Open No. 80986/1995). A polymer containing a polar group such as a hydroxyl group, which has high crystallinity and high polymer cohesive energy density, is used as the material for this coating film. Specific examples thereof include polyvinyl alcohols and ethylene-vinyl alcohol copolymers. The adoption of a laminated structure of a vapor deposited layer and a gas barrier coating film can improve the gas barrier properties and can solve the above problems.
Polar groups such as a hydroxyl group and an amide group, however, are likely to bond to water molecules, and the gas barrier properties are deteriorated as the environmental humidity enhances. That is, when the contents of a pack formed of the film are a water-containing liquid or a water-containing food, the gas barrier properties are deteriorated due to the influence of water vapor or the like in the contents, disadvantageously leading to deteriorated quality of the contents during storage.
Further, for some foods, after filling food into a pack, the packed food is generally subjected to sterilization with hot water by boiling or retorting. The above gas barrier material, however, undergoes a deterioration in gas barrier properties and a deterioration in mechanical strength such as adhesive strength during the treatment and thus is not suitable for use in the treatment method.